Known in the art is a process for producing a powder-like iron-sodium tartrate complex comprising reacting ferric chloride FeCl.sub.3 with sodium tartrate Na.sub.2 C.sub.4 H.sub.4 O.sub.6 in an aqueous medium at a molar ratio of the components of 1:3.0-3.5 respectively, a concentration of the ferric salt of 0.1 to 0.4 mol per liter of the starting reaction mixture and a pH of the reaction mixture about 14.2; the reaction is carried out in the presence of a stabilizing agent--sodium sulphite, glucose, galactose or hydrazine. Then from the resulting reaction mass the desired product is precipitated by means of a settling agent such as methanol, ethanol, butanol or acetone at a volume ratio of the settling agent to the reaction mass of 0.25-5.0:1.0 respectively. However, the stabilization agent and sodium chloride forming in the course of the reaction of sodium tartrate with ferric chloride are precipitated along with the desired product, wherefore the latter is contaminated.
Then the desired product together with the impurities is separated from the mother liquor and dried to give a powder-like iron-sodium tartrate complex of a green colour which contains the above-mentioned impurities (cf. U.S. Pat. No. 4,265,675 Int. Cl. C 13 K 1/02).
This prior art process has an essential disadvantage residing in that the resulting desired product contains the above-mentioned impurities. As it is well known, the presence of such impurities in the iron-sodium tartrate complex considerably lowers the dissolving power of an aqueous-alkaline solution of this complex in respect to cellulose. For this reason, the cellulose solvent obtained with the use of the powder-like iron-sodium tartrate complex prepared by the above-discussed process is less effective, since dissolution of cellulose therein is very slow (more than 24 hours) and incomplete. Such solvent cannot be used for analysis of the above-mentioned characteristics of cellulose.
Also known in the art is a process for preparing a powder-like iron-sodium tartrate complex comprising reacting ferric nitrate Fe(NO.sub.3).sub.3 with sodium tartrate Na.sub.2 C.sub.4 H.sub.4 O.sub.6 in an aqueous medium at the molar ratio of the above-mentioned reactants of 1.0:3.0 respectively, concentration of the ferric salt of 0.5-0.7 mole per liter of the starting reaction mixture and at a pH thereof having value equal to or exceeding 11. The desired product is precipitated from the reaction mass by ethanol at a volume ratio of the precipitation agent to the reaction mass equal to 1.0-3.0:1 respectively. In doing so, an impurity of sodium nitrate formed as a result of interaction of the starting components is precipitated together with the desired product. The precipitate comprising the desired complex and the impurity of sodium nitrate is in the form of a viscous oily liquid of a green colour. Then the green viscous oily liquid is separated from the mother liquor, whereafter the impurity of sodium nitrate is partially separated from this oily liquid. To this end, the liquid is diluted with water in the ratio of 1/3 volume of water per one volume of the oily liquid. The diluted oily liquid is treated with ethanol in the proportions specified hereinbefore. As a result, a residue is again obtained in the form of a green-colour viscous oily liquid, but with a smaller content of sodium nitrate. The abovementioned operations of dilution of the oily liquid with water, treatment with ethanol and separation of the precipitate from the mother liquor are repeated for two more times. This results in partial removal of the sodium nitrate impurity from the viscous oily liquid, the impurity passing into the mother liquor. Then water is removed from the oily liquid by way of a repeated treatment thereof with ethanol. As water is progressively removed from the oily liquid, the latter transforms into a glass-like product wherefrom a further removal of water by treatment with ethanol becomes substantially hindered. For this reason, the glass-like product is dried over P.sub.2 O.sub.5 under vacuum. After drying the product is finely divided to give a pure particulated green iron-sodium tartrate complex (cf. Reyon Zellwole und andere Chemiefasern, Nr. 1, published January 1956 (Offizielles Organ der internationalen Chemiefaservereinigung, Berlin), Georg Jayme, Werner Bergmann "Uber die vereinfachte Herstellung eines Losungmittels fur Zellulose auf der Basis eines Eisen-Weinsaure-Natrium Komplexes," s. 27-29).
A disadvantage of this prior art process resides in that it involves too many stages. Furthermore, this process necessitates the use of vacuum, P.sub.2 O.sub.5 and high rates of ethanol consumption--30-40 ml of ethanol per gram of iron-sodium tartrate complex. The yield of the desired product in this process is only 75% of the theoretical. The desired product prepared by this process is hygroscopic and requires special storage conditions. The process does not make it possible to prepare a complex of iron with sodium tartrate which would have a reproducible composition, since the above-mentioned water washing employed for the removal of the impurity of sodium nitrate from the oily liquid results in a partial (with different degree) hydrolysis of iron.
The process is hardly implementable, it is impossible to be performed on a commercial scale.